A feedback control system is a system that operates to achieve prescribed relationships between selected system variables by comparing functions of those variables and using the comparison to effect control. System variables are those quantities or conditions of the system which are subject to change. Examples of such variables include an electrical voltage level generated by an amplifier or physical force applied to a specimen by a servoactuator. Control is the governing of the response of a controlled subsystem, e.g. rotational velocity of an electrical motor, strain in a structural member of a truss, or position of an elevator.
Feedback control systems are conceptually divided into two portions, the controlled subsystem and a controlling subsystem. The controlling subsystem manipulates the controlled subsystem. The elements of the controlling subsystem can include reference input elements, forward controlling elements, feedback elements and a summing point or analogous operational element. Reference input elements are transfer elements which receive and modify an applied command signal into a form which serves as a standard of comparison for the directly controlled variable, i.e. the measured response of the controlled system. Put another way, the reference input element, for a steady-state command signal, for fixed forward controlling elements and a fixed controlled subsystem, allows adjustment of the response of the controlled subsystem to desired values. The output of the reference input element is called the reference input signal.
The reference input signal is applied to a feedback control loop. A comparator element or summing point in the feedback loop receives the reference input signal and compares it to a feedback signal for generating an error or difference signal. Forward controlling elements operate on actuation signals. The forward controlling elements may include an element for tuning of the feedback loop and includes an element for varying a directly manipulated variable applied to the controlled subsystem. The forward controlling elements provide conversion of electrical signals operable on by the control system into a directly manipulated variable, such as force, for application to the controlled system. The forward controlling elements may also add energy to the system. An example of a forward controlling element is a servohydraulic actuator, used to generate a force vector for application to a specimen or to move an object.
The controlled subsystem of the feedback loop responds to the directly manipulated variable. Strain on a structural member in a body receiving the force is an example of such a response. A particular response of interest is the output response. The output response of interest of the controlled subsystem is measured by a feedback element to generate the feedback signal. Feedback elements include transducers such as pressure sensors or strain gauges, which generate an electrical signal proportional to the output response. The feedback signal is returned to the summing junction or element. The summing junction, forward controlling elements, controlled subsystem and feedback element are an independent control loop, and operate as a subsidiary control system within another control system.
Among tens of thousands of contemporary uses of feedback control is its use in fatigue testing of specimens. Fatigue in a specimen is a function of the prolonged imposition of cyclic stress on the specimen. Application of a periodically varying load on a specimen is one way of generating mechanical stress. An engineer interested in fatigue in one or more structural members of an aircraft wing can select a measurable variable, such as strain, as a way of recording the history of fatigue development in the member. Actuators receiving actuation signals generate variable and static loads for imposition on a specimen. Responsive strain in various members can then be measured by strain gauge transducers attached to the members. The measurements are monitored as outputs and provide feedback to the controlling system to govern the process.
Engineers may desire that the output response follow a predetermined set of values. In the example of the aircraft wing given above, it might be desired that strain imposed on the selected member follow a predetermined function. To achieve this goal, manual tuning of the feedback control system has been required. Tuning is an adjustment in relation to frequency of the system to secure optimum performance here to produce a feedback signal having the desired function. However, any change in frequency or amplitude of the command signal, or to the static component of the command signal, or change occurring in the controlled element, has, in most prior art systems, necessitated retuning.
The foregoing discussion has dealt primarily with the effects on a feedback control system of changes in operating conditions or in system elements transfer functions. External disturbances can also effect any system variable. While such disturbances do not, strictly speaking, change the transfer function of the system, they can affect system accuracy in following the desired function values. Modifications to feedback control systems which promote system stability are, all other factors being equal, beneficial.
Other factors can also influence the extent to which the directly controlled variable exhibits the desired function. Under the influence of an experiment, physical changes can occur in the controlled subsystem which change its transfer characteristics over time. Such changes affect the output response generated by a particular actuation signal and have necessitated repeated retuning of a control system in some experiments.
When tuning a control system for a given performance level, control engineers have typically required knowledge of the amount of gain and phase shift experienced by signals propagating inside the feedback loop. At certain critical frequencies these are known as the gain and phase margins of the system. These quantities, taken with certain simplifying assumptions, have been used by the engineer to determine what the controller gains must be to achieve a given performance level. If the control engineer has not been able to analytically derive the gain and phase margins from physical principles, he has had to measure them directly. Typically, the measurements have been taken by observing control loop signals on an oscilloscope. A more automated approach to measurement is taken by Hagglund et al, U.S. Pat. No. 4,549,123, wherein a specialized controller called a "relay autotuner" is connected to the system allowing the gain margin to be measured directly. However, Hagglund requires self-oscillation before the essential process quantities can be measured, and Hagglund does not measure phase margin, only gain margin.